The present invention relates to a lighting device.
Lighting devices, such as fluorescent lamps and light emitting diodes (LED elements), are generally designed to increase photopic luminance under well-lit conditions (photopic vision). This allows for cones, which perceive brightness with photopic vision, to function. A higher photopic luminance allows for humans to perceive light as being brighter.
However, when humans use mesopic vision under a situation such as on a street lit by lampposts during the nighttime, rods function in addition to cones. Cones have a spectral luminous efficiency with a peak wavelength at 555 nm, and rods have a spectral luminous efficiency with a peak wavelength at 507 nm. Thus, an increase in only the photopic luminance would have a low effect. Such a shift of the spectral luminous efficiency is known as the Purkinje effect.
Japanese Laid-Open Patent Publication No. 2008-91232 describes an example of a lighting device including a plurality of light sources so that the cones and the rods in the retina are both effective when using mesopic vision. At least one of the light sources is formed to have a peak wavelength between 450 to 550 nm. This light source has a wavelength range that includes 507 nm, which is the peak wavelength for the spectral luminous efficiency of the rods.
The lighting device of Japanese Laid-Open Patent Publication No. 2008-91232 improves vision under mesopic conditions by coping with the Purkinje effect. However, for example, when the snow cover is present on a road, the surface of the road becomes more whitish than when there is no snow. This changes the reflectivity of the road surface. As a result, the Purkinje effect may not occur. In such a case, the lighting device of Japanese Laid-Open Patent Publication No. 2008-91232 would not improve vision because the Purkinje effect does not occur.
Further, the lighting device designed to cope with the Purkinje effect emits bluish light due to the relatively large number of short wavelength components included in the light. Accordingly, there is still room for improvement in vision with the lighting device that is designed to cope with the Purkinje effect.